DC power is used to power both the audio and the signalling circuit extending between a subscriber's telephone and the Central Office call-handling equipment in the telephone system. In such systems, a modulated DC current carries the voice messages that pass over the telephone lines. The use of DC current is a carry over from the first telephone systems which utilized batteries to supply the necessary current to energize the telephone lines. In the telephone systems in use today, AC power is converted to DC power to energize the telephone lines, and DC batteries are used only in a backup capacity for the AC power source. However, the technique of energizing the telephone lines with DC power is still referred to as "battery feed."
The interface between a telephone subscriber and the Central Office in most telephone systems is a magnetic hybrid transformer. As the telephone system call handling equipment uses separate wires for the receive and transmit signals, and as a combined received and transmit signal is sent to the telephone subscriber on a two wire "loop," the magnetic hybrid transformer provides the 2:4 wire conversion between the telephone and the Central Office call handling equipment. Additionally, the magnetic hybrid transformer supplies the telephone lines with the DC power. Of these two functions, the 2:4 wire conversion is the one around which the magnetic hybrid transformer was designed. The battery feed function was adapted to work with the magnetic hybrid transformer.
The magnetic hybrid transformer is an entirely passive device utilizing coils of wire wound about a magnetic core. With the increasing integration of many of the elements in a telephone system into small electronic circuits, the magnetic hybrid transformer is an attractive candidate for which an integrated circuit replacement would be desirable. Several configurations utilizing integrating circuits persently exist to implement the 2:4 wire conversion property of the magnetic hybrid transformer. However, previous attempts to integrate into these chips the battery feed function have met with less than total success.
The problems encountered by previous attempts to integrate both the battery feed and 2:4 wire conversion in an integrated circuit arise from the fact that the battery feed function is a purely DC characteristic, and the 2:4 wire conversion process is entirely an AC function. Great difficulty has arisen in implementing these dual functions in a single integrated circuit. Additionally, the telephone system has a standard AC impedance. This impedance also must be implemented along with the battery feed, and 2:4 wire conversion features. The combining of these three features has presented insurmountable problems in the previous attempts to integrate them into a single circuit.
Another difficulty in integrating the magnetic hybrid transformer arises from the fact that the telephone system utilizes special amplification circuitry to service those telephone subscribers located far from the Central Office. These special amplification circuits are called loop extenders. The loop extender is basically a current amplifier which utilizes the amount of DC current on the line to control the degree of AC amplification of the combined transmit and receive signals. Even those previous attempts to integrate all of the functions of the magnetic hybrid transformer into a single chip which have been partially successful have not managed to operate in a compatible fashion with the loop extender circuitry.
Accordingly, it is the principal object of this invention to simulate the DC characteristics of the magnetic hybrid transformer.
It is a further object of this invention to simulate the direct current characteristics of the magnetic hybrid transformer in a manner which will allow a single integrated electronic circuit to provide both the battery feed and 2:4 wire conversion functions of the magntic hybrid transformer.
It is a final object of this invention to achieve compatibility of an active circuit replacement for the magnetic hybrid transformer with the loop extender circuitry used in the telephone system.